Footing Concrete Calculator
Concrete for strip footings running in a trench, or for individual pad footings.
Your result
Enter your footing dimensions above to see how much concrete you need.
Footing width, depth and reinforcement depend on your soil classification, the loads above and the relevant standards. This tells you how much concrete fills a trench of the size you describe. It cannot tell you whether that size is right, and getting a footing wrong is expensive to correct once the building is on it.
Strip footings and pad footings
A strip footing is a continuous run of concrete in a trench, carrying a wall along its whole length. You measure the total length of the trench, its width and its depth.
Pad footings are separate blocks, each carrying a point load — a post, a pier, a column, a deck stump. You measure one pad, then say how many you have.
The calculator keeps these apart deliberately. In strip mode the length you enter is the whole run, so a number in the count field is ignored rather than multiplied in. That mistake would silently multiply your order, and you would not notice until a truck arrived with far too much concrete.
Measuring a strip run that turns corners
Measure along the centre line of the trench, not the outside edge. Following the outside edge counts each corner twice and over-orders. If the footing has several separate runs, add their lengths together and enter the total.
Worked examples
Strip footing for a small extension. A 20 m total run, 450 mm wide, 500 mm deep: 20 × 0.45 × 0.5 = 4.5 m³. With a 10% allowance that is 4.95 m³, and you would order 5.0 m³.
Pad footings for a deck. Six pads, each 600 × 600 × 600 mm: 0.216 m³ each, 1.3 m³ in total. At roughly 110 bags per cubic metre for a 20 kg premix, that is around 145 bags — enough that ready-mix starts to look attractive, if a truck can reach.
Things that change how much you actually need
- Trench walls collapse and bulge. A trench dug in sandy or loose soil ends up wider than the spade, and every extra millimetre of width runs the whole length. This is where the waste allowance earns its keep.
- Depth varies along the run. Trenches are rarely a uniform depth, and any soft spots get dug deeper.
- Stepped footings. On a slope, footings step down in stages. Work out each step as its own run and add them.
- Founding depth. You have to reach suitable material. If it is deeper than expected, the volume goes up with it.
Frequently asked questions
I have three separate strip runs. Do I use the count field?
No. Add the three run lengths together and enter the total as the length. The count field applies to pad footings only, and is ignored in strip mode precisely so it cannot triple your order by accident.
Does this cover the thickened edge of a slab?
The perimeter beam of a slab is handled by the slab calculator, which works out the slab and its edge together. Use this one for internal beams under load-bearing walls, then add the two figures.
What about round holes for fence posts?
Use the post hole calculator. It works on the diameter of a round hole rather than a rectangular pad, and can subtract the volume the post itself takes up.
Do I subtract the reinforcement bar?
No. Trench mesh and bar displace very little concrete relative to the trench, and far less than the waste allowance covers.